The development of the neural system has been an intensively studied area. For example, neurodegenerative disease has become an important concern due to the expanding elderly population which is at greatest risk for these disorders. These diseases, which include Alzheimer's Disease, Multiple Sclerosis (MS), Huntington's Disease, Amyotrophic Lateral Sclerosis, and Parkinson's Disease, have been linked to the degeneration of neural cells in particular locations of the central nervous system (CNS), leading to inability of these cells or the brain region to carry out their intended function. Therefore, it is desirable to find out how neural cells, including neurons, oligodendrocytes and astrocytes, are generated. With such findings, neural cells can then be produced in vivo or in vitro to compensate for the degenerate or injured neural cells.
A major progress in this study was the discovery of multipotent neural stem cells (for example see U.S. Pat. Nos. 5,750,376; 5,980,885; 5,851,832). Briefly, these stem cells may be isolated from both fetal and adult brains, and cultured in vitro indefinitely. These cells retain the ability to proliferate in response to growth factors, or differentiate into all lineages of neural cells (neurons and glia cells, including astrocytes and oligodendrocytes) in response to differentiation stimuli. To date, epidermal growth factor (EGF), transforming growth factor alpha (TGF-α) and fibroblast growth factor-2 (FGF-2) are the only factors known to induce the proliferation of single precursor cells that can give rise to neurons, oligodendrocytes, and astrocytes. However, the role of other regulatory factors or cells in the development of the neural system remains to be uncovered.